1) Flippases
d. Translocate lipids from outer (extracellular) leaflet to inner (cytosolic) leaflet
They are ATP dependent enzymes and they translocate phospholipids against a concentration gradient.
2) Floppases
f. Move phospolipids from inner (cytoplasmic) leaflet to outer (extracellular) leaflet
c. ABC transporter
They move substrates such as phosphatidylcholine and cholesterol against their concentration gradient.
3) Scramblases
a. Not ATP-dependent
b. Activation may result in increased membrane lipid symmetry
e. Translocate phosphatidylserine, preventing apoptosis and engulfment by macrophages
g. Move phospohlipids across the lipid bilayer down the concentration gradient
Their enzymatic activity is bidirectional and depends on the calcium concentration.